WALNUT CREEK (CBS 5) — A Walnut Creek police officer has been arrested for having sex with an underage girl, according to authorities.

Officer Shawn Wallace was arrested February 3rd and was booked into the Contra Costa County jail in Martinez on a charge of unlawful sexual intercourse with a minor between the ages of 16 and 18 without force, according to Gene Greenwald, Chief of Inspections with the Contra Costa County District Attorney’s office.

Officer Wallace is now free after posting bail.

Wallace also was booked on two other charges: oral copulation with a person under 18 without force and dissuading a witness.

“He is accused of discouraging the witness from cooperating with authorities,” Greenwald said.

Greenwald said he could not elaborate on the circumstances of the crime or the victim’s age.

Walnut Creek Police Chief Joel Bryden said Officer Wallace was a patrol officer at the time of the alleged incident, but that he resigned from the police department on February 3rd, just before his arrest.

He said Wallace was in law enforcement for 20 years.

Chief Bryden said he hoped the arrest would not reflect poorly on his department.

“We uphold the highest standards and we have an excellent police department,” said Bryden, who called the arrest “discouraging.”